A polytetrafluoroethylene fine powder is produced by coagulating an aqueous emulsion of polytetrafluoroethylene (hereinafter referred to as PTFE) obtained by polymerization of tetrafluoroethylene (hereinafter referred to as TFE) in an aqueous medium by using an emulsifier, so-called an emulsion polymerization method, to prepare a polytetrafluoroethylene fine powder (hereinafter referred to as PTFE fine powder) in a wet state, and then drying it. Since the PTFE fine powder is likely to be fiberized when shearing force is applied, it is molded by a special method, and then used for various purposes.
As one of the molding methods of the PTFE fine powder, paste extrusion molding has been employed (for example, Non-Patent Document 1). That is, to the PTFE fine powder, a lubricant such as naphtha or a petroleum hydrocarbon having a dry point of at least 100° C. is added and uniformly impregnated to obtain a mixture. Then, the mixture is preliminary molded to a desired shape such as a tubular form to prevent fiberization of the PTFE fine powder and then charged to an extrusion cylinder and subjected to plastic deformation by a ram to obtain an extrusion molded product. Then, the extrusion molded product is heated in a drying oven to remove the lubricant by evaporation and sintered in a heating oven, whereby a desired molded material can be obtained. Otherwise, the extrusion molded product is subjected to rolling by rollers before the lubricant evaporates to obtain a sheet or a film, and then, after removal of the lubricant, the sheet or the film is subjected to stretching at a low stretching rate, to obtain an unsintered green tape. Further, by monoaxially or biaxially stretching the unsintered green tape at a high stretching rate at a high speed in a heated condition, a high strength porous film or sheet can be obtained.
Usually, with regard to the PTFE fine powder, the lower the extrusion pressure at the time of paste extrusion, the higher the productivity or yield of the product, and therefore the low extrusion pressure property is desired for the PTFE fine powder.
As a method to obtain a PTFE fine powder having a low extrusion pressure property, Patent Document 1 discloses applying radiation such as γ-ray to the PTFE fine powder.
Further, as a method to obtain a PTFE fine powder having a low extrusion pressure property, a method is known wherein the drying temperature is set at a low temperature during a drying step of the PTFE fine powder in a wet state which is obtained by coagulating an aqueous PTFE emulsion after emulsion polymerization. For example, Patent Document 2 discloses that an extrusion pressure as low as possible can be achieved in paste extrusion molding by conducting drying at a low temperature of from about 40° C. to about 80° C. Further, Patent Document 3 discloses the method to increase the content of a lubricant to be used at the time of paste extrusion.